1. Field of the Invention
The present invention relates to a method and an apparatus for determining characteristics of a semiconductor thin film.
2. Description of the Related Art
For example, in regard to manufacture of a thin film transistor for a switching element of, e.g., a liquid crystal display apparatus or an organic electroluminescence display apparatus, M. Hatano, S. Moon, M. Lee and K. Suzuki, (C. P. Grigoropoulos), Journal of Applied Physics, vol. 87, No. 1, 2000, pp, 36 to 43, Excimer Laser-Induced Temperature Field in Melting and Resolidification of Silicon Thin Films reports a method by which an annealed position provided to an amorphous silicon thin film is irradiated with a monitor light ray and an intensity of its reflected light is detected, thereby determining characteristics of the thin film.
The above cited reference mentions that the reflected light of the monitor light from the silicon thin film is detected by, e.g., a silicon PN junction photodiode type photodetector which has a response time of 1 nano-second (which will be denoted as “ns” hereinafter), i.e., a time resolution of 1 ns, and a temporal change of a detection signal waveform is measured by a sampling oscilloscope which samples a frequency signal of 1 GHz.
The silicon thin film is molten by laser irradiation for several-ten to 100 nm and crystallized in a subsequent solidification process, and a growth of crystal grains is generated. As a result, the silicon thin film varies from the amorphous type to the polycrystal type. A time required from melting to end of solidification is several-hundred ns.
The silicon thin film is affected to have metallic properties due to melting, an extinction coefficient k is increased, a reflected light intensity is thereby increased, the extinction coefficient k is decreased due to solidification after melting, and the reflected light intensity is thereby decreased. The temporal change of the reflected light intensity of the silicon thin film during melting or solidification is detected by the photodetector, the characteristics of the thin film are determined, and the crystallinity of the thin film is evaluated based on the characteristics.
However, in the method described in the above cited reference, only one set of information is obtained every 1 ns with respect to the reflected light intensity.
For example, a time of melting, a reflection factor or a transmittance is obtained from this one set of information, and it is thus difficult to determine important optical characteristics of the thin film such as a refractive index or an extinction coefficient in order to evaluate a degree of progress that the thin film is crystallized.
Further, it is substantially impossible to measure a change in the reflected light intensity concerning a melting-solidification process of several-hundred ns, i.e., a degree of progress that the thin film is crystallized with a time resolution higher than 1 ns.
Therefore, the characteristics of the thin film cannot be correctly specified in the prior art, and there is known that a defect of, e.g., electrical characteristics is generated in a liquid crystal display apparatus or the like using as a switching element a thin film transistor having an unsurely evaluated thin film.